Tea kettle preloaded energy chunks have a gap

[chrisklus]

Found when fixing #176.

After the tea kettle is steaming and then energy chunks are turned on, there is a gap between the initial stream of chunks and the chunks that follow out of the burner.

@arouinfar and I think that the preloaded energy chunks should originate from the burner instead of the center of the tea kettle (which would close the gap), but this is lower priority than other issues.

[jbphet]

This behavior is also present in the Java sim.

[chrisklus]

This is fixed, closing.

[KatieWoe]

This can happen to a minor degree if you switch tabs and come back. For phetsims/qa#293

[KatieWoe]

Second unintended consequence: I noticed that when turning on energy chunks you can get a situation where chunks appear to be coming from an empty bucket.
https://drive.google.com/file/d/1a2IiTkoh0xKLUbtXJzNztsXVtA2AcQSj/view?usp=sharing

[chrisklus]

#178 (comment) has been fixed by capping the max dt for a time step.

[chrisklus]

#178 (comment) has also been fixed. Now, if the burner is on when chunks are being preloaded, the chunks originate from the burner, but it the burner is off when they're being preloaded, then they originate from the tea kettle.

[chrisklus]

@KatieWoe could you please verify my changes above on master and close if all looks good?

[KatieWoe]

I would say this looks pretty good on master.

[KatieWoe]

Looks good in 1.1.0-rc.2